This invention relates to furnaces for and the melting of metals, and alloys of metals ("metals" unless otherwise noted), for treating and/or alloying the metals.
High performance metals, such as titanium, are routinely melted and are treated to give them desired physical and/or chemical characteristics. For example, substantially pure titanium (Ti) and Ti alloys are used for a variety of high performance applications ranging from aircraft turbine rotor blades to golf club heads and beyond. Titanium must be melted at a high temperature while it is being treated, for example to adjust its oxygen content to 0.16-0.18 weight percent of O.sub.2 to give it optimal strength. During treatment, care must be exercised to prevent the contamination of the titanium by other substances. This is accomplished by melting it in an inert atmosphere, such as argon, while subjecting it to heat generated, for example, by a plasma torch which forms an electric discharge arc from an electrode of the torch to a molten pool of the metal contained within a cooled metallic crucible in which the metal is located. Other metals are treated according to the type of metal and the characteristic(s) one wishes to attain.
In the past, such metals were melted in relatively large furnaces, holding, for example, as much as 5000 lbs. Such furnaces have a crucible inside a sealed enclosure that is closed with a removable port. When a plasma torch is used as the heat source, it has been movably mounted to the enclosure top surface so that the electrode can be moved towards and away from the bottom of the crucible and can further be swiveled or otherwise moved to deflect it in a lateral direction, for example along a conical path, so that the electric arc and plasma discharge of the torch can be swept over the pool of molten metal in the crucible. Such movably mounted plasma torches provide excellent heating but are expensive to manufacture, install and maintain.
The cost of such torches and manipulators is nevertheless justifiable because relatively large batches of metal can be melted at a time. The pressure within the enclosure is kept relatively high, typically in the order of 250 torr to 860 torr. The torches are axially movable into closer proximity to the crucible surface for striking and maintaining the needed electric arc, because at the prevailing, relatively high pressure in the enclosure, only relatively short arc lengths can be maintained.
The walls of the crucibles in such furnaces are constructed of electrically and thermally highly conductive metal, such as copper, and are usually water-cooled to keep them from melting or contaminating metal being melted. After the treatment, the molten metal is gravitationally drained through consumable ceramic or graphite nozzles into molds located beneath the crucible. The consumable nozzle is typically heated to the melting point of the metal by an auxiliary source such as an induction coil and susceptor.
After the metal has been poured, the plasma torch is turned off, the furnace is permitted to cool, the port or cover is opened, and a skull of the metal that has been melted (a thin metal layer that hardens over the inside surface of the crucible) is removed. Thereafter, the ceramic or graphite nozzle is inspected and replaced if necessary.
Considerable time necessarily elapses between the melting of successive batches of metal because, following each pour, the furnace must cool down sufficiently to permit its opening, the removal of the skull, inspection of the nozzle, and its recharging with a fresh load of metal to be melted. This is acceptable when the metal is melted in large, e.g. 5000 lb., batches. For the same reasons, the high cost of prior art furnaces presented no particular obstacle because of the high price obtainable for specialty metals such as titanium, which in its properly treated and purified state presently yields prices of as much as $6/lb.
However, enterprises which require relatively smaller quantities of such metals, such as the needs of golf club head manufacturers for substantially pure titanium, could not afford to acquire or operate prior art furnaces. In many cases, they have to purchase the material in ingot form. The ingots are remelted and cast into golf club head molds. This is a relatively expensive manufacturing operation and, additionally, generates a great deal of titanium scrap which can be sold at only a fraction of the cost of the ingot price, say in the range of between 60.cent.-80.cent./lb. Material costs therefore heavily contribute to the relatively high cost of such golf clubs.